Final answer:
Corrosion is the greatest threat to the soundness of metals and the structural integrity of an aircraft.
Step-by-step explanation:
The greatest threat to the soundness of metals and the structural integrity of an aircraft is corrosion.
Corrosion is the process of metal being gradually destroyed due to chemical reactions with its surrounding environment. In the case of aircraft, exposure to moisture, salt, and other corrosive substances can cause the metal to weaken, leading to the formation of cracks, pitting, and ultimately structural failure.
To mitigate the effects of corrosion, aircraft are regularly inspected, treated with protective coatings, and undergo maintenance procedures such as cleaning, painting, and surface treatment.